The present invention relates to loop coupled YIG resonators, and more particularly to an improved loop coupled YIG resonator which reduces parasitic series inductance to increase the usable frequency range.
A typical YIG resonator 10 is shown in FIGS. 1 and 3. A metallic housing 12, split into two halves 12a and 12b, has a thinned central section 14. In the central section 14 are located one or more resonator cavities 16, each having a YIG resonator 18 located centrally therein. At each end of the housing 12 is a connector 20 having a conducting lead 22 lying in a groove milled in the two halves 12a, 12b of the housing. The conducting lead 22 tapers down to a fine wire 24 near the cavities 16. A coupling loop 26 is located in the cavity 16 with a one-half turn around the YIG resonator 18. The ends of the coupling loop 26 are integrally attached to the fine wires 24 on one end and are grounded at the other end 27. A pair of electromagnets 28, having a central pole piece 30 and a surrounding electrical winding 32, are located contiguous to the housing 12 so the tips of the pole pieces contact the central portion 14 of the housing 12 to provide the necessary magnetic field for the YIG resonators 18. The two spheres 18 are coupled together by an interstage pair of loops 25 as shown in FIG. 2.
For the configuration as shown in FIG. 3 the equivalent circuit at very high frequencies is shown in FIG. 4. The resonator 18 is represented by an LC tank circuit and the actual loop portion of the coupling loop 26 is represented by an inductor which acts as the primary of a transformer to couple energy to the inductive component, i.e., secondary, of the LC tank circuit. The ends of the coupling loop 26 which extend from the actual loop to the walls of the cavity 16 are represented by additional series inductive elements. Due to the effect of the parasitic series inductive elements the coupling efficiency between the coupling loop 26 and the YIG resonator 18 decreases at higher frequencies. This decreased coupling efficiency becomes significant at 18 GHz and above as shown in FIG. 5 by the solid line. The series inductance could be reduced by moving the cavity walls closer together, but this results in a decreased Q of the resonator.
Therefore what is desired is a means for minimizing the parasitic series inductance without affecting the Q of the resonator.